1. Field of the Invention
The present invention relates to substrate holding units, exposure apparatus, and device manufacturing methods, and more particularly to a substrate holding unit that holds plate-shaped substrates, an exposure apparatus that comprises the substrate holding unit serving as a holding unit for a substrate subject to exposure, and a device manufacturing method using the exposure apparatus.
2. Description of the Related Art
Conventionally, in a lithographic process for manufacturing devices such as a semiconductor device or a liquid crystal display device, exposure apparatus were used that transfer a pattern formed on a mask or a reticle (hereinafter generally referred to as a “reticle”) onto a substrate such as wafer or a glass plate (hereinafter generally referred to as a “wafer”) coated with a resist or the like, via a projection optical system. In recent years, due to higher integration of semiconductor devices, sequential moving type projection exposure apparatus are being widely used, such as a reduction projection exposure apparatus (the so-called stepper) based on a step-and-repeat method or a scanning projection exposure apparatus (the so-called scanning stepper) based on a step-and-scan method.
In these projection exposure apparatus, a wafer stage is provided that can move freely in a two dimensional plane, and a wafer is held by a wafer holder fixed on the wafer stage, by means such as vacuum chucking or electrostatic suction.
However, wafers and reticles are held by substrate holding units, which in turn are mounted on movable bodies such as a stage. Therefore, the total weight including the substrate holding unit and the movable body greatly influences position controllability (including position setting controllability) during the motion of the movable body. Therefore, for example, in the publication of Japanese Patent No. 3000361, a proposal is made for reducing the weight of the movable body on which the reticle is mounted, by providing holes (pockets) on a front surface side, a rear surface side, or the inside of the movable body.
However, it is difficult to simply apply the art disclosed in the above publication of reducing the weight of the movable body where a reticle holder is provided to a wafer holder, since the reticle holder holds the reticle only on both edges, whereas in the wafer holder, a vacuum chucking method is employed to hold the wafer fixed to the wafer holder. Therefore, in a conventional wafer holder, when holes are formed, the holes naturally have to be provided on the front surface side. If the holes are provided on the rear surface side or in the inside, vacuum chucking of the wafer will be difficult. And, even if holes that do not have any influence on the vacuum chucking are provided in the inside, the effect they will have in weight reduction will be small. That is, when holes are formed in a wafer holder, it is naturally a requirement for weight reduction, for the holes to be in common usage with holes for vacuum chucking.
In addition, in order to hold the wafer by suction while maintaining its flatness, the surface of the wafer holder requires an extremely high degree of flatness. Therefore, in the manufacturing process for making wafer holders, in the final process, a surface polishing process is included.
However, no matter how precisely and flat the surface is polished, when the shape and the size of the suction holes are determined without any consideration of vacuum chucking, inconvenience occurs, such as the wafer warping during vacuum chucking, the surface accuracy of the wafer not being secured due to the wafer surface reflecting the uneven shape of the surface of the wafer holder, or furthermore, a decline in rigidity. In addition, variation of rigidity in the holes formed for weight reduction purposes causes processing variation and step variation, and furthermore, only holes which sectional shape is a ring or holes that have only a small influence on the rigidity can be formed, which in turn produces only a small effect in weight reduction.
In addition, the recent requirements for shorter exposure wavelength and higher numerical apertures (N.A.) both lead to a narrower focal depth of the projection optical system, and when clamping of a foreign material occurs, it causes unevenness on the surface of the wafer, which is highly likely to cause a focal shift, leading to a degrading in transfer accuracy.
To prevent the clamping of foreign materials as much as possible, recently, for example, as is disclosed in Japanese Patent Laid-open No. 01-129438, a wafer holder based on a pin chuck method is relatively frequently used. With this wafer holder, a large number of pin-shaped support members (hereinafter referred to as “pins”) support the wafer, and by using such a wafer holder occurrence of the above clamping of foreign materials can be suppressed almost without fail.
FIG. 18A shows a planar view of an example of a conventional wafer holder similar to the one disclosed in the above publication. A wafer holder 25′, shown in FIG. 18A, comprises: a base member 26′ having a circular plate shape; multiple pins 32′ arranged on the upper surface of base member 26′ (a surface closer to the page surface in the drawing) with equal space; and a ring-shaped protruded portion (hereinafter referred to as “rim portion”) 28′. And, multiple pins 32′ and rim portion 28′ support the wafer from below, as well as hold the wafer by suction, such as by vacuum chucking.
In the conventional wafer holder described above, for example, when the wafer was supported with the pins by vacuum chucking or the like, arrangement and space and the like of the pins were set so that the deformation amount of the wafer was within a predetermined permissible amount at positions where the pins did not support the wafer.
In addition, in order to secure the degree of flatness of the wafer (substrate), the contact portion of the wafer holder to the wafer is preferably flush to the wafer. Therefore, in the manufacturing stage of making the wafer holder, a precision polishing is performed on the upper surface of the pins and the rim portion, using a polishing unit.
In wafer holder 25′ shown in FIG. 18A, or in the wafer holder disclosed in the above publication, in order to avoid clamping of foreign materials in between the pins and the wafer as much as possible, a contact ratio of the pins was set at a minimum within the limits of the permissible range of deformation in the above description. On the other hand, the rim portion results in exceeding a constant width, due to limitations in the polishing technique. Accordingly, the proportion of the contact area of the rim portion becomes large in the total contact ratio, when the contact ratio of the pins set based on the amount of the above deformation is small.
In addition, when a method of setting the contact ratio of the pins related to the above conventional art is employed, during the polishing process in the manufacturing stage, the contact pressure of the pins per unit area with respect to the polishing portion (processing portion) of the polishing unit is larger when compared with the rim portion. Also, the pins have sufficient coverage of abrasive grains used during polishing, whereas in the rim portion the coverage of abrasive grains tends to be insufficient. Therefore, the processing speed of the pins is faster when compared with the rim portion. And, due to this difference in the processing speed, in between the pins and the rim portion, as is shown in FIG. 18B, which shows an end view of a vertical section of wafer holder 25′ in FIG. 18A holding a wafer W by suction, a processing step Δa is made in between rim portion 28′ and pins 32′. For such reasons, a deformation having an angle of inclination θ is formed in the free edge portion of wafer W, in the vicinity of the outer edge portion, which in turn causes defocusing to occur during exposure. Especially with the recent trend of trying to obtain chips from the portion close to the outer edge of the wafer, deformation in the outer edge of the wafer as in the above description may become a cause a decrease in yield of devices serving as an end product.
Meanwhile, since exposure apparatus are used for mass production of microdevices such as a semiconductor device, requirements on improving throughput (processing capacity) are pressing, on a par with, or even with more pressure than improving the exposure accuracy, and it is beyond question that one of the key points is to improve the performance of the wafer stage, especially to improve the positional controllability which includes higher speed, higher acceleration, and higher position setting accuracy.